Thesis (Ph. D.)--University of Hawaii at Manoa, 1993.Microfiche.xi, 123 leaves, bound 29 cmThe hepatopancreas, or digestive gland, of Homarus americanus (and other crustaceans), has been shown to playa major role in the digestion and absorption of nutrients. Although the hepatopancreas has been implicated in the manufacture and release of digestive enzymes there have been no reports of its ability to secrete other solutes such as ions. The organ has been suggested as a possible site of excretion due to its cellular morphology and direct access to the lumen of the digestive tract. It has been demonstrated that sulfate is present in lobster hemolymph at a concentration lower than in seawater. The present investigation utilized isolated membrane vesicles to determine the mechanisms and characteristics of sulfate transport across both the apical and basolateral membranes of the hepatopancreas. The brush border membrane of the hepatopancreatic epithelium appears to contain a transport protein which translocates cytoplasmic sulfate in exchange for luminal chloride. This antiport mechanism operates in an electrogenic fashion by exchanging one sulfate for one chloride, resulting in the movement of a net negative charge out of the epithelial cell. There was no indication of sodium-sulfate cotransport commonly reported for the brush border membrane of vertebrate renal and intestinal epithelia. It was found that the antiporter was stimulated by the presence of a high concentration of luminal protons. This suggested that the sulfate antiporter was regulated by the brush border sodium-proton exchanger which acidifies the hepatopancreas lumen. Sulfate was found to exchange for the dicarboxylic anion oxalate by an electroneutral antiporter in the hepatopancreatic basolateral membrane. This transporter would allow for movement of sulfate from the hemolymph, which bathes the hepatopancreas, to the cytoplasm of the epithelium. The serosal antiporter did not show a response to manipulation of proton concentrations. A model of transcellular sulfate secretion by the hepatopancreatic epithelium has been proposed utilizing the two antiporters working in sequence to bring about vectorial sulfate movement. This is the first experimental evidence which implicates the crustacean hepatopancreas as a secretory organ
